1. Field of the Invention
The present invention relates to a measuring device for measuring the height or thickness of a workpiece such as a semiconductor wafer held on a chuck table included in a processing machine such as a laser processing machine.
2. Description of the Related Art
In a semiconductor device fabrication process, the upper surface of a substantially disk-shaped semiconductor wafer is formed with a plurality of crossing separation lines called streets to thereby define a plurality of partitioned regions arranged like a matrix, and a plurality of devices such as ICs and LSIs are formed in these partitioned regions. The semiconductor wafer is cut along these streets to separate the partitioned regions, thus fabricating individual semiconductor chips.
As a method of cutting such a semiconductor wafer or optical device wafer along the streets, a laser processing method is known. In the laser processing method, a pulsed laser beam that can pass through the wafer is used. The pulsed laser beam is applied to the wafer so as to be focused on the region to be cut inside the wafer. More specifically, a pulsed laser beam having a wavelength of 1064 nm, for example, is applied to the wafer from one side thereof so as to be focused inside of the wafer along each street, thereby continuously forming an altered layer inside the wafer along each street. By forming the altered layer inside the wafer along each street, the strength of the wafer is lowered along each street. Accordingly, by applying an external force to the wafer along each street, the wafer is cut along each street (see Japanese Patent No. 3408805, for example).
As another method of cutting a platelike workpiece such as a semiconductor wafer, a method of forming a laser processed groove is known. In this method, a pulsed laser beam is applied to the workpiece along each street to thereby form a laser processed groove along each street. The workpiece is cut (broken) along each laser processed groove by using a mechanical breaking device (see Japanese Patent Laid-open No. Hei 10-305420, for example). Also in such a case of forming the laser processed groove on the workpiece along each street, it is important to locate the focal point of the laser beam at a predetermined height in the workpiece.
As a method of forming a via hole at a position where an electrode called a bonding pad is formed on the upper surface of a semiconductor wafer, it is known that a pulsed laser beam is applied to the lower surface of the semiconductor wafer so as to form the via hole reaching the bonding pad. If there are variations in the thickness of the semiconductor wafer, the via hole reaching the bonding pad cannot be accurately formed. It is accordingly necessary to accurately grasp the thickness of the semiconductor wafer at the position where the bonding pad is formed.
However, a platelike workpiece such as a semiconductor wafer has undulations, causing variations in thickness. As a result, it is difficult to perform uniform laser processing. In the case of forming an altered layer in the wafer along each street, variations in the thickness of the wafer cause a problem such that the altered layer cannot be uniformly formed at a predetermined depth in the wafer in relation to a refractive index in applying a laser beam to the wafer. Also in the case of forming a laser processed groove on the wafer along each street, variations in the thickness of the wafer cause a problem such that the laser processed groove cannot be formed with a uniform depth. Further, in the case of forming a via hole reaching the bonding pad formed on the wafer, variations in the thickness of the wafer cause a problem such that the via hole reaching the bonding pad cannot be accurately formed.